Human and simian immunodeficiency viruses exhibit remarkable biological diversity in their tropism and cytopathicity that is likely relevant to their pathogenesis in vivo. Experimental models are needed in which these biological differences can be defined genetically and their effects evaluated in biochemical, structural and mechanistic terms. The investigators propose to continue studies of an experimental model for SIV in which a biological variant, termed CP-MAC, was derived from an infectious molecular clone. Compared to the parental virus, CP-MAC was shown to exhibit remarkable structural and biochemical differences that were determined by mutations the viral env gene. Their studies during the first funding period have led to the novel observation of an endocytosis motif in the viral transmembrane (TM) cytoplasmic domain that is likely conserved among all HIVs and SIVs and that may have important implications for the pathogenesis of these viruses in vivo. They propose to continue a comprehensive evaluation of this finding as well as other biological and structural aspects of this virus in 3 major areas: (1) Efforts will continue to define the viral determinants responsible for CP-MAC s altered tropism, its increased CD4 binding affinity, and the increased stability in the association of its SU and TM proteins. Experimental approaches include the construction of viruses and env- expression vectors that contain chimeric and mutated env proteins. (2) Cellular interactions of the CP-MAC env proteins will be evaluated, including: structural determinants in the TM cytoplasmic domain involved in regulating env protein trafficking to and expression on the cell surface; functional differences in the env protein that occur post-CD4 binding; and a cellular molecule that may be functioning as a CD4 co- receptor for this virus. 3) Finally, the relevance of these findings to other SIV and HIV-1 models will be evaluated. In particular, they will utilize similar approaches to consequences of mutations in these domains. Additional efforts will characterize biological effects in molecularly-derived SIVs that are pathogenic in animals so that their in vivo consequences can be evaluated in ongoing and future studies. Their in vitro model is well suited to address mechanisms involved at the viral and cellular levels and may help to establish paradigms regarding the cellular and virologic basis for biological diversity and the mechanisms by which these viruses adapt to different cell types and target tissues during infection.